1. Field of the Invention
The present general inventive concept relates to system power switching and, more particularly, to system and method of switching power provided to an electronic aircraft system.
2. Description of the Related Art
Aircrafts utilize a combination of mechanical and electrically based instrumentation to convey the state of the aircraft to the pilot or flight crew. Mechanically based instruments typically use fluid pressure, such as air, oil, and fuel, to cause deflections in needles located within instrument housings located in the cockpit to indicate the corresponding state of a system on the aircraft. Electrically based instruments typically use solid-state sensors to convert mechanical forces into electrical signals, which can then be displayed on various types of equipment located within the cockpit.
Aircraft also make extensive use of electronic instrumentation systems for purposes of navigation, communication, and surveillance.
Smaller, lighter aircraft types, which are typically referred to as “general aviation aircraft”, have electrically based systems are typically powered by a 14 or 28 volt direct current (“DC”) power source. These power systems typically include a battery, a power generation mechanism attached to the aircraft's engine(s), and a voltage regulator. Energy provided by the running engine is used to both power electrical devices on the aircraft and maintain the charged state of the battery. When the engine is not running, energy is obtained solely from the battery. The regulator is used to maintain a constant voltage level regardless of the engine speed. A single, sealed lead-acid battery, a regulator, and an automotive style alternator is a commonly employed configuration.
The alternator functions by converting mechanical energy created by the engine into electrical energy by spinning the shaft on the alternator, which creates electrical power by inducing voltages and currents into coil windings. During normal engine operation, this electrical power is regulated to a voltage such as 13.2 to 14.4 volts, which is slightly higher than the normal quiescent voltage of a lead acid battery. This elevated voltage level is sufficient to charge the lead acid battery. When this charge voltage is removed. i.e. the engine stops or the alternator fails, the lead acid battery voltage returns to a slightly lower voltage such as 12.6 to 12.8 volts.
Because the battery is directly tied to the overall electrical supply system for the aircraft, the voltage of the overall electrical system of the aircraft directly reflects whether the alternator is actively capable of generating power based entirely on the voltage of the aircraft's electrical system. This voltage can be used as an indicator of the aircraft's electrical system health, and indicate whether a failure has occurred.
Because of the relatively high energy density requirements and the mechanical nature of the engine and alternator, aircraft with this type of electrical system are highly susceptible to loss of electrical generation ability. Wiring, alternator, and regulator failures are a fairly regular occurrence in general aviation aircraft. This is especially true for older aircraft that are retrofitted with additional electrically based avionics that place an additional strain on very old electrical components.
For single engine aircraft, loss of the one engine is also sufficient to cause loss of electrical generation capability. For twin engine aircraft, loss of both engines is typically required for loss of electrical generation capability. In multiple engine aircraft, loss of electrical generation capability is typically related to common-mode failures, and may occur due to fuel starvation, fuel contamination, or other factors. Some aircraft support an air-driven electrical generator, but this configuration is not common given the additional weight, cost, and complexity of these systems.
When power generation capability is lost, the aircraft's electrically based systems must be powered solely from the aircraft's battery. When this occurs, it is a common practice to reduce or shut down power to or “load-shed” non required systems. This is implemented by turning off such systems such as entertainment equipment and other non-essential loads such as cabin lights, and air conditioning. Items that are required for the continued safe flight and landing of the aircraft are considered “essential loads” and cannot be turned off under these circumstances.
The objective of the load-shedding exercise is to reduce the overall power consumption of the aircraft such that essential loads or critical systems can continue to operate until such time that the aircraft can be safely landed. Thirty (30) minutes is industry-accepted minimum time period for general aviation aircraft. To achieve this time objective, the aircraft manufacturer must design the capacity of the electrical charge system, battery size, and wiring in consideration of the required loads for a given avionics system installed on the aircraft.
The architecture of these aircraft systems makes it more difficult to add electrically based equipment to the aircraft, and a new load analysis must be performed to ensure that adequate battery power reserve is still provided as each new piece of equipment is added. Performing a load analysis is very difficult and time consuming. It may also result in the requirement to increase the size of the battery, which may increase the size, cost, and weight of related components.
Modification of the battery may be required especially in the scenario where essential systems that are electrical in nature are being added. This is typically encountered when a mechanical instrument is being replaced by its electrical equivalent. In this scenario, an additional electrical load is being added to the aircraft, but for a piece of equipment that cannot be load-shed as it is an essential instrument. The replacement of the primary mechanically based attitude, airspeed, and altimeter with an electronic equivalent is an example of this scenario.
Accordingly, there is a demand for a system that allows for the replacement of essential and/or non-essential mechanically based instrumentation with more modern electronically based equipment without effecting the safety margins designed into an aircraft's electrical power generation system, and does not require a new load analysis, upgrading of the aircrafts electrical system, or provisioning for a larger battery.